Radio frequency (RF) spectrum is a scarce resource. In cellular or personal communications systems environment, an increasing number of users need to be simultaneously serviced while also attempting to avoid interference among such users. One way to increase the number of simultaneous users on a given frequency band is code division multiple access (CDMA). CDMA refers to a form of multiplexing which allows numerous signals to occupy a single transmission channel thereby optimizing the use of available bandwidth. The term CDMA is sometimes more generally referred to as direct sequence (DS) pseudo-noise (PN) modulation, or DSPN. The term “direct sequence spread spectrum” (DSSS) may also be used. For convenience of simplified explanation, the terms CDMA, DSPN, and DSSS are interchangeably used herein. This technology is generally used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.
CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum signal processing. Input data is first digitized for transmission, and the transmitted signal is then made to vary according to a defined pattern or code. CDMA signals can generally be intercepted by a receiver that is programmed with the same code as the transmitted signal so that such receiver follows the transmitter code. Spread-spectrum modes support several times as many signals per unit bandwidth as typically found in analog modes.
In multi-code (MC) CDMA radio telephone systems, several CDMA signals associated with a single user may be sent between two stations in order to provide the user with a higher data rate. In such systems, each CDMA signal associated with a single user may be spread using orthogonal Walsh codes. However, the transmission quality of the CDMA signals in such systems typically degrades when the signals travel over more than one, path between the transmitter and the receiver. This is because such “multi-path propagation” causes co-channel interference between the transmitted CDMA signals. Multiuser communications systems that employ CDMA exhibit a limit on the number of users that can simultaneously communicate over a channel and maintain a specified level of performance per user. This limitation is caused by the domination by other user interference over the additive thermal noise.
The CDMA codes are generated by, e.g., Walsh functions that mathematically form an orthogonal set. Thus, any two Walsh functions are orthogonal to each other, and signals encoded with two separate Walsh functions should cause no mutual interference when they are time aligned. However, because multiple signals often are not time aligned, complete orthogonality is not achieved in practice. As a result, interference between otherwise orthogonal signals occurs. This is known as multiple access interference (MAI).
CDMA/DSPN communications generally involve a use of wider bandwidths than other forms of communications. Because the spectrum is generally a finite resource, increasing signal clutter makes finding large contiguous portions of spectrum available for wide bandwidth signals difficult. Furthermore, an allocation of communication system bandwidth generally results in an inefficient use of the spectrum, with guard bands and spectrum gaps that may otherwise be used for data transmission.
In view of the foregoing, it is desirable to provide spread spectrum communication that allows for efficient and greater spectrum aggregation and management. In addition, it is desirable to provide spread spectrum communication having interference mitigation that allows for an “underlay” communications network to effectively coexist in a non-contiguous fashion with an existing legacy communications network without materially interfering with performance. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.